I am currently looking for an inexpensive keg system. I would appreciate any advice on where to purchase one online.
I would also like to know where I can get the co2 tanks filled, and that approximate cost.

I purchased refurbished Pepsi-Cola "Cornelius" 5-gallon kegs from E-Bay for a good price. In fact, if you go out there now and search for "cornelius keg" you will see many for sale for only 10 bucks. That is a great price. Your homebrew supply store should have inexpensive adapters, hoses, etc. to connect your keg(s) to a CO2 tank and a tap. These kegs are great and the best part is that you no longer have to wash bottles! As for the CO2 tank, you should be able to get it recharged and any welding supply store. A 5 lb tank can be filled for as little as 10 bucks and will carbonate and serve about a dozen kegs on one charge. Hope this helps.
Cheers,
Tom
"Beer is proof that God loves us and wants us to be happy."
-Benjamin Franklin